The present invention relates to a piston seal formed of a rubber composition, and a disc brake using the piston seal.
A piston seal is generally formed of a rubber composition containing rubber as an essential component. For example, a vehicular disc brake is equipped with a caliper body including a piston and a cylinder. A piston seal is provided in a ring-shaped groove formed in the inner circumferential surface of the cylinder. In the disc brake, brake pads are pressed against a disc rotor secured to each wheel by applying hydraulic pressure, and rotation of the wheel is stopped by friction force of the brake pads which are made of a friction material. The piston seal has a role of sealing brake fluid and a role of returning (rolling back) the piston which has been moved forward by applying hydraulic pressure. Applying hydraulic pressure moves forward the piston that has been inserted into a cylinder bore. This causes the brake pads to be pressed against the disc.
Specifically, the piston seal enables the cylinder and the piston inserted into the cylinder bore to fluid-tightly and movably come in contact with each other. The piston which has been moved forward by applying hydraulic pressure is rolled back by the piston seal (see Japanese Patent Publication No. H03-59291). Therefore, the piston seal is required to have toughness for securely sealing the brake fluid and elasticity for returning (rolling back) the piston which has been moved forward by applying hydraulic pressure.
The temperature of the caliper body of the disc brake increases during the operation due to frictional heat occurring between the disc rotor and the brake pads. Therefore, the piston seal is also subjected to high temperature. The piston seal formed of a rubber composition thermally expands at high temperature, and the modulus of elasticity of the piston seal decreases. In this case, the amount of rollback of the piston changes due to thermal expansion and a decrease in modulus of elasticity of the piston seal, whereby the allowance of the brake effects change. In a disc brake for a motorcycle, the amount of stroke of the brake lever changes, whereby the driver feels that the brake operation is unnatural.